Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≤min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system may support time division duplex (TDD) and/or frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the base station to extract transmit beamforming gain on the forward link when multiple antennas are available at the base station. In an FDD system, forward and reverse link transmissions are on different frequency regions.
The main focus of the traditional LTE design among other things is the improvement of spectral efficiency, ubiquitous coverage, enhanced QoS (Quality of Service) support, and the like. This typically results in high end devices, such as the state-of-art smart phones, tablets, etc. However, low cost, low rate devices need to be supported as well. Some market projections show that the number of low cost devices may largely exceed number of today's cell phones.
A study item on provision of low-cost MTC (machine type communications) UEs (User Equipments) based on LTE was done in LTE Rel-11. Particularly, the items under study included reduction of maximum bandwidth, single receive RF (Radio Frequency) chain, reduction of peak rate, reduction of transmit power, and half duplex operation.
Since the intended data rate for the low cost devices is less than 100 kbps, it is possible to operate these devices only at narrowband width, for example, to reduce costs. Two operation scenarios may be considered for the deployment of low cost devices. One straight-forward deployment scenario is to set aside some narrow bandwidth, e.g. 1.25 MHz, to support the MTC operations. In this scenario, no or little standard changes may be necessary for such operations. Another, more interesting deployment scenario is to operate low cost UEs in a large bandwidth. In this case, low cost UEs may co-exist with regular UEs. Two further possible scenarios may be considered for operation of low cost UEs in a large bandwidth. In one scenario, low cost UEs may operate over the whole available bandwidth (e.g., up to 20 MHz). This scenario may not have any impact on the standards, but it may not be helpful in reducing cost and battery power consumption. In another scenario, low cost UEs may operate over a small portion of the bandwidth.